In a chimeric molecule, two or more molecules that exist separately in their native state are joined together to form a single molecule having the desired functionality of all of its constituent molecules. The constituent molecules of a chimeric molecule may be joined by chemical conjugation or, where the constituent molecules are all polypeptides, they may be fused together to form a single continuous polypeptide. If one of the constituent molecules is a ligand, then the resulting chimeric molecules bind to cells bearing receptors specific for the particular ligand.
Where the first constituent molecule is a ligand and the second protein is a cytotoxin, the chimeric molecule may act as a potent cell-killing agent specifically targeting the cytotoxin to cells bearing a particular receptor type. For example, chimeric fusion proteins which include interleukin 4 (IL4) or transforming growth factor (TGF.alpha.) fused to Pseudomonas exotoxin (PE) or interleukin 2 (IL2) fused to Diphtheria toxin (DT) have been tested for their ability to specifically target and kill cancer cells (Pastan et al., Ann. Rev. Biochem., 61: 331-354 (1992)).
Alternatively, where the ligand is attached to another specific binding moiety such as an antibody, a growth factor, or another ligand, the chimeric molecule may act as a highly specific bifunctional ligand. This ligand may act to bind and enhance the interaction between cells or cellular components to which the chimeric molecule binds. Thus, for example, where the chimeric molecule is a fusion protein in which a growth factor is fused to an antibody or antibody fragment (e.g. an Fv fragment of an antibody), the antibody may specifically bind antigen positive cancer cells while the growth factor binds receptors (e.g., IL2 or IL4 receptors) on the surface of immune cells. The fusion protein may thus act to enhance and direct an immune response toward target cancer cells.
Ligands are typically employed in chimeric molecules to act as specific targeting moieties. Generally it is desirable to increase specificity and affinity and decrease cross-reactivity of the chimeric molecule to make it more effective. For example, native PE and DT are highly toxic compounds that typically bring about death through liver toxicity. PE and DT can be transformed into chimeric toxins by removing the native targeting component of the toxin and replacing it with a different specific targeting moiety (e.g. IL4 which targets cells bearing IL4 receptors). However, even these chimeric toxins show some non-specific binding. They attack the liver in addition to their target cells and, when given in large doses, may also produce death due to liver toxicity.
It has been observed that growth factors, and other targeting moieties, frequently show lower specificity and affinity for their targets when they are incorporated into chimeric molecules such as fusion proteins. See, for example, Debinski, et al., J. Biol. Chem., 268: 14065-14070 (1993); Lorberboum-Galski, et al., J. Biol. Chem., 263: 18650-18656 (1988); Williams, et al., J. Biol. Chem., 265: 11885-11889 (1990); and Edwards, et al. Mol. Cell. Biol., 9: 2860-2867 (1989).